The invention relates generally to the field of image reconstruction. In particular, the invention relates to a technique for defining prior information for multi-spectral images. More particularly, the invention relates to a technique for generating a multi-spectral image based on the prior information.
An imaging system typically operates by projecting X-ray beams from an X-ray source through an attenuating object, such as a patient. The X-ray beams may be collimated between the source and the object into a fan or cone shape, depending on the configuration of the detector optimal patient exposure, or other factors. The attenuated beams are then detected by a set of detector elements. The detector elements produce signals based on the intensity of the X-ray beams. The measured data are then processed to represent the line integrals of the attenuation of the object along the ray paths. The processed data are typically called projections. By using reconstruction techniques, such as filtered backprojection, cross-sectional images are formulated from the projections. Adjacent cross-sectional images may be displayed together to render a volume representing the imaged region of the object or patient.
As will be appreciated by those skilled in the art, the linear attenuation coefficient of a material is a function of two separate events that may occur when an X-ray beam passes through a material. The first event is known as Compton scatter and denotes the tendency of an X-ray photon passing through the material to be scattered or diverted from the original beam path, with a resultant change in energy. The second event is known as photoelectric absorption and denotes the tendency of an X-ray photon passing through the material to be absorbed by the material.
As one might expect, different materials differ in their scatter and absorption properties, resulting in different attenuation coefficients for different materials. In particular, the probability of Compton scattering depends in part on the electron density of the imaged material and the probability of photoelectric absorption depends in part on the atomic number of the imaged material, i.e., the greater the atomic number, the greater the likelihood of absorption. Furthermore, both the Compton scattering effect and photoelectric absorption depend in part on the energy of the X-ray beam. As a result, materials can be distinguished from one another based upon the relative importance of the photoelectric absorption and Compton scattering effects in X-ray attenuation by the material. In particular, measurement of the attenuation produced by a material at two or more X-ray energy levels or using two or more energy spectra, i.e., multi-energy or multi-spectral CT, may allow for respective Compton scattering and photoelectric absorption contributions to be quantified for a material.
Since the decomposition of the attenuation into Compton scattering and photo-electric absorption contributions is not a well defined process, it would be desirable to model the spectral dependence of an image based on prior knowledge about the contribution of Compton scattering and photoelectric absorption in a material. In addition, it would be desirable to use this prior knowledge to generate a multi-spectral image with improved image quality.